Technical Field
The present disclosure relates to an AC motor driving method and an AC motor driving system. More particularly, the AC motor driving method and the AC motor driving system of the present disclosure have a capacitor having a small capacitance value.
Description of Related Art
An industrial motor drive device is designed to obtain a wide constant power operation, low output voltage and current harmonic, high power density and low cost. The commonly used methods can be summarized as follows. One is the field-weakening control that reduces the motor magnetic flux, and the other is DC voltage boosting under the same input supply voltage. The DC voltage boosting is simpler, more effective and can improve the power quality, but the cost is too high. FIG. 1 is a schematic view showing a conventional three-phase AC motor driving system with two-stage circuits. In FIG. 1, a step-up AC-DC converter 401 is in series with an AC driving circuit 402. The step-up AC-DC converter 401 combined with the AC driving circuit 402 can provide bi-directional control of power flow, high power factor and high step-up ratio DC voltage, but the main drawback is its high cost. FIG. 2 is a schematic view showing a conventional three-phase AC motor driving system with three-stage circuits. In FIG. 2, a step-up circuit 404 is in series with a three-phase full-wave rectifier circuit 403 and an AC driving circuit 405. This conventional three-phase AC motor driving system with three-stage circuits can provide high power factor and high step-up ratio DC voltage, and its cost is lower than the system in FIG. 1.
In FIGS. 1 and 2, a capacitor C having a larger capacitance value is used, and enhancing the efficiency of the motor driving system is not considered in FIGS. 1 and 2. Since the capacitor C is used as a filter and usually is an electrolytic capacitor with large capacitance. Its lifetime duration often decreases because of the evaporated electrolyte. Except for that, the power transistors of the AC driving circuit are operated independently and have no interaction to the other stage circuits. An integrated AC motor driving system and method providing higher power factor, higher efficiency and longer lifetime is still commercially desirable.